I can help a bit.
1. Setting up user accounts can be accomplished in one of two ways. Either they must be made a local unix system user, or you must setup virtual mailbox hosting using some form of a backend(ldap, sql, db3, etc...). It's up to you to choose. The easiest is to setup local user accounts, but unfortunately it's the toughest to administrate since you have security concerns to worry about with more users on the console of the box. Virtual is my choice using a mysql backend.
2. I assume postfix is running standalone? If you've added your services to the /etc/xinetd.d/ directory and /etc/services file and it's still not running then the command set you're using is probably invalid. Make sure you haven't fubar'd a switch or accidentally set it to disable, or something small like that.
3. There is no all in one solution. Love it or hate it this will be the culmination of at least two, if not 3 applications. Postfix for SMTP handling, 3rd party for pop3 and 3rd party for imap.
is the howto I used to setup my virtual hosting. It's obviously gentoo centric, but shouldn't be too tought to muck through getting the sources compiled. You obviously don't have to use everything they suggest, but it's nice to have all the frills working.